This invention relates to all-wheel drive systems for motor vehicles and, more particularly to an improved arrangement for accommodating a controllable freewheeling clutch in a housing which is rigid with both the housing of the vehicle rear axle drive and the propeller shaft torque tube.
The U.S. Pat. No. 4,605,087 issued Aug. 12, 1986, to Ashauer et al. discloses an all-wheel drive system for vehicles wherein the front and rear driving wheels are connected permanently by a drive train between the front transaxle and the rear driven axle. A freewheeling or overrunning clutch device is provided in the driveline for uncoupling the drive train in accordance with the applied torque. Thus, a decrease in the rotational speed of the front wheels can not be transmitted to the rear wheels through the driving connection between them. Accordingly, when the primary or foot brake is actuated with a rigid coupling between the front and rear wheels, the overrunning clutch system responds to the braking action on the front wheels uncoupling the rear wheels whereby any overbraking of the front wheels can not be transmitted to the rear wheels through the drive train.
The Ashauer drive train further provides a liquid friction or viscous coupling connection located at the input end of the rear axle differential for transmitting variable torques depending upon the speed differentials between the input and output shafts of the drive train. Thus, when the directly driven front wheels and rear wheels rotational speed is about equal the viscous coupling will transmit practically no torque allowing the rear wheels to follow the front wheels with no significant driving action. When front wheel speed exceeds the rear wheel speed, such as "wheel slip" caused by wet, snow-covered or icy roads, the viscous coupling will react and produce a rising high torque transfer connection between the drive train input and output shafts.
In one embodiment of Ashauer the rear differential housing includes a rigid longitudinal tube portion adjoining the liquid friction coupling. The tube portion, however, is relatively short and is secured to the frame at a location which is spaced a relatively small distance in front of the axis of the rear axle drive such that objectional rear axle drive torque reaction forces are imparted to the vehicle body.
The Ashauer patent discloses an overrunning clutch directly connected to the housing of the front axle drive with resultant engine vibrations being transmitted via the front axle drive to the clutch housing. This arrangement creates vibrations which may interfere with the actuating means for locking the overrunning clutch. In addition it may be impractical to arrange the overrunning clutch close to the front axle drive. This is particularly true in vehicles that have originally been designed for a front wheel drive transaxle and subsequently are converted to an all-wheel drive mode.
A four-wheel drive system is provided in the VW "Golf Syncro" automobile which has a transverse front engine that requires a bevel drive to the longitudinal propeller shaft. The rear axle differential has a transmission ratio of almost 1, meaning that the speed of the longitudinal shaft as almost equal to that of the half shafts. Drive into the rear axle is achieved by bevel gears. The Golf Syncro has no central differential but rather a viscous coupling flange-mounted on the rear axle differential that provides a permanent, variable four-wheel drive known as "Viscous Transmission". A free-wheel or overrunning clutch is housed in the rear axle that provides complete disconnection of the rear axle in overrun condition and when breaking. An electropneumatic system automatically locks the overrunning clutch when reverse gear is selected, meaning that the four-wheel drive is also available for reversing.